DESCRIPTION This proposal tests the hypothesis that a secreted form of the beta-amyloid precursor protein (sAPPalpha) plays a role in the development of hippocampal neural circuitry by activating specific receptors linked to a signal transduction pathway that modulates neuronal excitability. The specific aims are (1) To test the hypothesis that sAPPalpha reduces excitability and responses to glutamate by activating a membrane associated guanylate cyclase linked to cGMP production and modulation of K+ channels and NMDA receptors. Studies will be done in cultured hippocampal neurons using whole cell perforated patch clamp analysis of K+ and NMDA induced currents, calcium imaging, analyses of neurotoxicity, assays of cGMP production and a battery of pharmacological tests (2) To define the structure function relationships for activation of the sAPPalpha signaling pathway defined in Aim 1. Recombinant sAPP deletion mutants and synthetic sAPP peptides will be employed (3) To isolate and clone a sAPPalpha receptor, sAPPR. Strategies will include screening already cloned "orphan" receptor guanylate cyclases, isolating and sequencing sAPPalpha-binding proteins, and a yeast interaction trap using LexA fused to sAPPalpha peptides to screen a human fetal brain cDNA fusion library. (4) To map the subcellular localization of the sAPPalpha signal transduction apparatus using immunocytochemical approaches. (5) To test the hypothesis that sAPPalpha regulates neurite outgrowth and synaptogenesis in developing hippocampal neurons via a cGMP-mediated mechanism. Neurite outgrowth and synaptogenesis will be evaluated in embryonic hippocampal cultures using established methods. Collectively these data will reveal the signaling mechanisms of a fascinating multifunctional protein that has been linked to neuronal survival and plasticity, and which may play a role in the deterioration of neuronal circuitry in Alzheimer's disease.